digraph G {
  a[label="merge [2, 4] [1, 6]", shape=rectangle];
  b[label="[2, 4]", shape=circle, style=invis];
  c[label="[1, 6]", shape=circle, style=invis];
  {rank=same; b,c}
  d[label="[4]", shape=circle, style=invis];
  e[label="[2]", shape=circle, style=invis];
  f[label="[6]", shape=circle, style=invis];
  g[label="[1]", shape=circle, style=invis];
  {rank=same; d,e,f,g};
  a -> b[style=invis];
  a -> c[style=invis];
  b -> d[style=invis];
  b -> e[style=invis];
  c -> f[style=invis];
  c -> g[style=invis];
}
